This invention relates to a catalyst for the oxidation of sulfur dioxide contained in an oxygen-containing gas current. The catalyst includes the following components:
A.sub.1) Vanadium oxide; and PA1 A.sub.2) Alkali metal oxide, alkali metal sulfate or mixtures thereof, as a catalytically active substance; and PA1 B) Silicon, aluminum, or mixtures thereof, in the form of oxides as a surface area enlarging component. PA1 A.sub.1) Vanadium oxide; PA1 A.sub.2) Alkali metal oxide, alkali metal sulfate or mixtures thereof, as a catalytically active substance; and PA1 B) Silicon, aluminum, or mixtures thereof, in the form of oxides as surface area enlarging components. PA1 C) Titanium oxide in anatase form, rutile form or mixtures thereof as a carrier material and ceramic binder.
In addition to street traffic, waste gases from combustion processes and industrial plants also constitute a source of the existing environmental pollution. The waste gases contain air pollutants, such as nitrogen oxides, carbon monoxide, hydrocarbons and sulfur dioxide. These pollutants are well known for their adverse impact on the environment.
If primary measures for the reduction of pollutant emissions have only a minor effect on the total emission of pollutants, then secondary measures, such as catalytic waste-gas cleaning methods, have to be employed.
Most of the combustion processes for fossil fuels takes place with fuel-air compositions which are leaner than stoichiometric. The catalytic waste-gas cleaning methods suitable for this purpose are the well known SCR (selective catalytic reduction) method for denitration by means of ammonia and the use of oxidation catalysts containing noble metals for the oxidation of carbon monoxide, hydrocarbons and also sulfur dioxide. The sulfur dioxide can be further processed to sulfuric acid. A combination of both methods is constituted by the method described in German Patent No. 36 01 378, with which method waste gases can be freed of NO.sub.x and SO.sub.2 with the creation of sulfuric acid.
For reasons of production technology, oxidation catalysts containing noble metals are not yet available in the necessary large geometric dimensions. It is still necessary to arrange individual, smaller monoliths over each other in order to obtain the necessary catalytic volume. Since dust settles out of dust-charged waste gases at the joints between adjacent monoliths, a great number of expensive dust blowers must be installed. In addition, the noble-metal component of these catalysts represents a considerable cost factor which can be tolerated in rather small waste-gas cleaning systems, but which results in prohibitive investment costs in rather large systems, such as heating power stations or superpower stations. Finally, the catalytically active noble-metal component is especially sensitive to poisonous components present in flue gas which can deactivate the catalyst.
As already explained, the catalysts for use with dust-charged waste gases are maintained clean by means of soot or dust blowers which swirl the dust deposited on the approach side of the catalysts and at the joints of catalytic packets by means of hot vapor or hot air. The dust is thereby returned to the current of waste gas which entrains it through the catalytic conduits, thus removing it out of the reactor. In order to reduce the number of cost intensive soot-blower devices in a catalytic reactor, it is necessary to have as few approach surfaces and joints as possible in reactors equipped with monolithic and/or honeycomb catalysts. A prerequisite for this is that the monolithic and/or honeycomb bodies should be produced with as great a piece length as possible. However, long monolithic and/or honeycomb bodies can not yet be produced in the case of coating catalysts containing noble metals.
Also, the use of soot or dust blowers functions only in the case of catalysts in monolithic or honeycomb form. Catalysts in bulk form can not be cleaned with the blowers, since the dust would only penetrate into deeper positions in the catalyst. The catalyst would fill with dust and the stoppage would remain. At the same time, there would be the danger of an uncontrolled turbulence of the bulk-material catalyst.
However, the noble-metal catalysts (e.g. platinum) used in the past to convert sulfur dioxide into sulfur trioxide, the anhydride of sulfuric acid, in the so-called contact method or especially base metal oxide catalysts (for example, vanadium pentoxide and alkali sulfate with silicon dioxide as carrier material) have been used exclusively in bulk form (as extruded blanks or rings). For the reasons explained above, this technology was problematic for the treatment of waste gases containing sulfur oxide, especially if the waste gases were charged with dust. Dust deposits had to be removed by means of sieving out the catalyst. The service life of such catalysts was therefore determined primarily by their mechanical strength.